1. Field of the Invention
The invention relates to the manufacture of optical fibre preforms by collapsing a sleeving tube onto a core rod and particularly, but not exclusively, to processes in which such a tube is collapsed onto a core rod during a fibre drawing process.
2. Description of the Related Art
In one known method of manufacturing an optical fibre preform, one or more sleeving tubes is collapsed onto a core rod placed within a sleeving tube. This method is described in U.S. Pat. Nos. 4,154,591 and 4,596,589. The method includes the steps of inserting the core rod into the sleeving tube and then slowly traversing a heat source along the length of the tube to collapse the tube onto the core rod. A suction force may be applied to the annular gap between the rod and the tube to assist the collapsing process.
Usually the melting of the sleeving tube onto the core rod takes place on a suitable lathe, followed by subsequent drawing of the optical fibre on a separate drawer tower. It has been recognised that carrying out these operations in two separate states has economic disadvantages and that it can be advantageous to collapse the sleeving tube onto the core rod and draw a fibre therefrom substantially simultaneously on one machine. A method for partially collapsing a sleeving tube onto a rod and then drawing the resultant preform in a continuous process on a drawing tower is disclosed in WO-A-9909437 (Pirelli Cavie Sistemi SpA).
JP-A-60155542 discloses another method in which a tube is collapsed onto a rod during the fibre drawing process. In this method, a rod and tube assembly is provided by positioning a rod in a tube such that an annular gap is defined between an outer surface of the rod and an inner surface of the tube. Adjacent end portions of the rod and tube are separately gripped by a holding device. The opposite ends of the rod and tube are fed into a heating furnace causing thermal collapse of the tube onto the rod. Downstream of the furnace, the newly collapsed portions of the rod are drawn to produce an optical fibre. The holding device is constructed to seal against the rod and the tube and defines a passage through which a suction force is applied to the annular gap to assist the collapse of the tube.
It has been recognised that an important aspect of the manufacture of optical fibre preforms and fibre production is the avoidance of the introduction of asymmetries in the preform geometry during fabrication and/or collapse of the preform into a solid mass, which asymmetries may be reflected in the cross-section of the resulting fibre, with consequent negative impact on the transmission properties of the fibre. In particular, it has been recognised that in order to avoid such asymmetries when manufacturing a simple two-layered preform by collapsing a tube onto a rod, particular attention should be paid in maintaining the tube centred onto the rod during the whole collapsing process. The applicant has recognised the need for a rod and tube assembly for optical fibre or optical fibre preform production that can be simply implemented and provide reliable centring of the tube.